What word can I use for a thing with positive and negative consequences? For example, taking a cab rather than driving has its advantages and disadvantages.
I'm not sure of a single word. You might use the phrase double-edged sword, e.g.
Taking a cab is a double-edged sword. You don't have to worry about parking, but it is more expensive.
For intermingled positive and negative emotions, I would use bittersweet. Clearly though, the phrase "taking a cab is bittersweet" doesn't really hit the mark, unless you're discussing the conflict of leaving vs going. The best I can come up with is "taking a cab rather than driving has mixed/conflicting/divergent consequences." I'll think more though, those still feel off the mark to me.
Dichotomy - when presented with the choices
Merriam-Webster defines Dichotomy as
something with seemingly contradictory qualities it's a dichotomy, this opulent Ritz-style luxury in a place that fronts on a
So in the current example, it could be used such as
Driving is a dichotomy with increased comfort but long term disadvantages.